Supporters of Russian communist party hold a rally in the Siberian city of Krasnoyarsk. Protests against the recent election results are growing across Russia. Photograph: Ilya Naymushin/REUTERS

Vladimir Putin
is set to face the biggest show of opposition yet to his strongman rule with tens of thousands of Russians promising to take to the streets on Saturday in a popular wave of discontent unseen since he came to power 12 years ago.

The opposition coalesced around a set of concrete demands, including the annulment of a parliamentary vote marred by fraud and the holding of new elections.

"We expect the biggest political demonstration of the last 10 years," said Ilya Ponomaryov, a Duma deputy with the Just
Russia
party and a protest organiser. "What will happen tomorrow is an important step in the development of our democracy."

More than 35,000 people indicated via Facebook that they planned to join the protest in Moscow. After a day of intense negotiations, protest organisers agreed to demands by the city government to move from Revolution Square to Bolotnaya (Swamp) Square, away from the Kremlin. Some protesters expressed concern that the site, on an island accessible by bridges, could be cut off by police.

The Kremlin has stepped up security around Moscow since the vote on Sunday, when Putin's United Russia gained nearly 50% of votes amid widespread evidence of falsifications and voting violations. Examples of stuffed ballots, paid votes and forged counts have continued to spread across the internet, feeding protesters' anger.

More than 50,000 police and 2,000 interior troops, backed by water cannon, helicopters and reinforced arrest lorries, have been deployed to the capital.

Protests were planned for more than 80 cities around Russia, including St Petersburg, where 12,000 indicated their intention to take part via VKontakte, a Russian social networking site.

Protesters hope to build on the momentum launched by a protest on Monday that saw up to 8,000 people turn out. Hundreds, including corruption fighter Alexei Navalny and youth leader Ilya Yashin, were arrested and remain in prison. Hundreds more were arrested in a subsequent protest the next day. "We demand the release of all political prisoners," said opposition leader Yevgeniya Chirikova.

Protesters promised to carry out a peaceful demonstration, with online activists urging people to show up with white flowers and white ribbons.

"These protests happening in Russia are the most civilised in the world," said Ponomaryov. "We haven't seen one overturned car, one broken window."

Alexander Gorbenko, deputy mayor of Moscow, warned protesters that police would crack down harshly on any violations. "If people are shouting slogans, marching, unfolding banners in an non-agreed place, that's called public disorder – and the police will harshly put a stop to it," he told Ekho Moskvy radio. Some protesters have pledged to gather at Revolution Square despite the change in the official location.

Gorbenko said he expected up to 7,000 people to show up at the protest. Popular cultural figures, including rapper Noize MC and writer Boris Akunin, who said he was flying in from Paris to attend the demonstration, are among those due to address the crowd.

The wave of protests has exposed simmering discontent among Russia's middle class, who have begun to decry the lack of democracy and freedom of expression under Putin and the runaway corruption that has flourished under his rule.

"Most of us who are out in the streets have achieved a decent level of personal comfort," said Dmitri Venkov, a 31-year-old film-maker. "Now we want the state to be a service to the people, not some kind of feudal lord. We're civilised, intelligent and educated … we deserve a normal, comfortable state."

Despite the authorities' decision to allow the protest, rumours of attempts to disrupt it abounded. Russian media reports said that schools and universities had declared classes for Saturday in an attempt to keep young people away. Gennady Onischenko, Russia's chief health official, warned protesters to say away at risk of getting the flu or SARS.

Some liberal websites faced further spam attacks on Friday, following a massive hack attack the day of Sunday's vote. Pavel Durov, the head of VKontakte, the Russian version of Facebook, said he had been called in for questioning by the Federal Security Service on Friday after refusing to submit to demands to shut down groups related to the protests.